The present invention relates to a suspension control apparatus for a vehicle.
As a conventional suspension control apparatus for a vehicle, there can be mentioned an apparatus comprising: a damper provided between a vehicle body and a vehicle wheel to generate a variable damping force; an actuator for controlling the damper; a lateral acceleration sensor for detecting a lateral acceleration of the vehicle body; lateral acceleration differential value detecting means for determining a lateral acceleration differential value by differentiating the lateral acceleration detected by the lateral acceleration sensor; a controller for calculating a control input for the actuator, based on the lateral acceleration and the lateral acceleration differential value and operating the actuator in accordance with the control input, to thereby control the damper to generate a desired damping force, so as to suppress rolling motion of the vehicle body. In order to calculate the control input for the actuator, the controller in this apparatus calculates a control input varying in proportion to the lateral acceleration and also calculates a control input varying in proportion to the lateral acceleration differential value. The controller adds these control inputs to thereby determine a total control input, and operates the actuator in accordance with the total control input.
Referring to FIG. 7, explanation is given below of a function of the above-mentioned controller, assuming that the vehicle rolls during turning, and a lateral acceleration a varying as shown in graph (a) of FIG. 7 has been detected by the lateral acceleration sensor. When the lateral acceleration a varying as shown in graph (a) of FIG. 7 has been detected, the controller calculates a control input for the actuator such as indicated by a dotted line T(.DELTA..alpha.) in graph (b) of FIG. 7, which varies in proportion to a lateral acceleration differential value .DELTA..alpha. calculated from the lateral acceleration .alpha.. The controller also calculates a control input for the actuator such as indicated by a dotted line T(.alpha.) in graph (c) of FIG. 7, which varies in proportion to the lateral acceleration .alpha.. Further, the controller adds these two control inputs, to thereby determine a total control input such as indicated by a dotted line T(S) in graph (d) of FIG. 7 [T(S)=T(.DELTA..alpha.)+T(.alpha.)]. The controller operates the actuator, based on the thus determined total control input varying as indicated by the dotted line T(S) in graph (d) of FIG. 7.
In many control systems of this type, the lateral acceleration .alpha. is mainly used for a feedback signal for a feedback (F. B.) system, while the lateral acceleration differential value .DELTA..alpha. is used for a feedforward signal for a feedforward (F. F.) system. With respect to the control input for the actuator, the control input on the basis of the lateral acceleration .alpha. and the control input on the basis of the lateral acceleration differential value .DELTA..alpha. are hereinafter frequently referred to as "F. B. control inputs" and "F. F. control input", respectively.
In the above-mentioned conventional suspension control apparatus, immediately after the start of rolling motion of the vehicle and just prior to the end of the rolling motion, the lateral acceleration differential value .DELTA..alpha. and hence the control input varying in proportion to the lateral acceleration differential value .DELTA..alpha. increases as indicated by the dotted line T(.DELTA..alpha.) in graph (b) of FIG. 7. Therefore, the total control input sharply increases immediately after the start of the rolling motion as indicated by the dotted line T(S) in graph (d) of FIG. 7. This leads to a rapid change in damping force, which imparts an occupant of the vehicle with queer and uncomfortable feel. It should be noted that in each of graphs (b) to (d) of FIG. 7, the control input is indicated above and below the abscissa because the directions of damping forces around at the time when the rolling motion starts and around at the time when the rolling motion ends are opposite (extension stroke and compression stroke).